1. Technical Field
The present disclosure relates to an image sensor and a method of manufacturing the same, and more particularly, to a complementary metal oxide semiconductor (CMOS) image sensor and a method of manufacturing the same.
2. Discussion of Related Art
In general, image sensors transform optical information into electrical signals. The image sensors include a CMOS type image sensor and a charge coupled device (CCD) type image sensor.
In the CCD type image sensor, each MOS capacitor is positioned adjacent to each other, and electron charges are stored into a MOS capacitor and move between MOS capacitors. In the CMOS type image sensor, the optical information is transformed into an electrical signal using a CMOS circuit, in which a control circuit and a signal processing circuit are formed in a peripheral region.
A unit pixel of the CMOS image sensor includes a photodiode for detecting light and a CMOS logic circuit for transforming the detected light into an electrical signal, thereby forming image data. The photodiode affects an optical sensitivity of the image sensor, and has been studied.
A dark level is an image defect in the CMOS image sensor. A dark current flows in the photodiode when the photodiode is not receiving any light due to electron charges being generated without a photoelectric reaction and accumulating in the photodiode. The dark current is mainly generated due to heat around a junction of the photodiode.
A plurality of hole and electron pairs are generated in the image sensor due to Joule heat caused by repeated usage of the image sensor. A plurality of crystal defects and dangling bonds are generated at a boundary portion of the active region and the field region since excessive damage and thermal and/or mechanical stress are created at the boundary portion when forming the field region. Electrons are accumulated around the crystal defects and dangling bonds. A portion of the accumulated electrons is diffused into the photodiode so that the electrons are accumulated into the photodiode. As a result, the dark current is generated due to the accumulated electrons in the photodiode and causes the dark level.
When an isolation layer in a field region is formed by a local oxidation of silicon (LOCOS) process, a boundary portion of the isolation layer is under excessive stress due to a thermal expansion during the oxidation of a substrate. The isolation layer in the field region has been formed through a shallow trench isolation (STI) process as a pixel size has been made smaller while maintaining sufficient oxide thickness for isolation. The STI process requires a dry etching process on a surface of a substrate. Due to the dry etching, the substrate in the STI process is under more excessive stress at a boundary portion adjacent to the isolation layer than the substrate in the LOCOS process. A surface of the substrate can be relieved of excessive stress by using an annealing process performed after the dry etching process. However, the dark level of a CMOS image sensor is not sufficiently reduced by the annealing process.
In a method of reducing the dark current, an image sensor is doped with P type impurities under a device isolation layer. When the P type impurities are heavily implanted under the device isolation layer, a hole and electron pair caused by heat is prevented from being diffused into the photodiode, thereby reducing the dark current. However, a portion of the electrons may be diffused into the photodiode despite the P type impurities so that a weak dark current is generated.
Accordingly, the dark current does not completely disappear in the image sensor despite the P type impurities. In addition, since the P type impurities must not contact the photodiode, a complicated manufacturing process is performed to make the image sensor.